1. Field of the Invention
The present invention relates to a technology for forming an image through an electrophotographic process by using an intermediate transfer member formed of an endless belt, with a capability of preventing an occurrence of a crack at an edge of the endless belt and an expansion of the endless belt.
2. Description of the Related Art
In an electrophotographic image forming apparatus, an electrostatic latent image formed on an image carrier (photosensitive member) based on optical image information, such as document reflected light, or electric image information from a host machine is developed by using toner supplied from a developing device as a toner image, and this toner image is transferred onto a transfer sheet and then fixed for image formation.
In a multicolor image forming apparatus that forms a full-color image by using toners of yellow, magenta, cyan, and black, it is generally the case that a scheme is adopted such that toner images of different colors formed on one or a plurality of image carriers are sequentially superposed on an intermediate transfer belt for transfer, thereby forming a superposed toner image, and then the superposed toner image on the intermediate transfer belt is transferred onto a transfer sheet for secondary transfer.
Also, an image forming apparatus has been known that is provided with a transfer belt that carries a transfer sheet to a transfer position when a toner image on an image carrier is transferred onto the transfer sheet.
Both the intermediate transfer belt and the transfer belt have a structure for extending an endless belt with extending rollers for running.
In this type of endless belt, stable and smooth running is important in increasing the quality of an image to be formed. For example, if the endless belt is deviated even slightly in a width direction or meandering, degradation in image quality is inevitable.
To get around this problem, as depicted in FIG. 4 for explaining a belt end supporting structure, a guide member 101 for preventing meandering is provided along the back surface of an edge in an width direction of an intermediate transfer belt (transfer belt) 100 as an endless belt, and an inner end of the guide member 101 is bumped against an end (step portion) 110a of an extending roller 110, thereby preventing meandering. That is, a deviation force occurring at the intermediate transfer belt 100 inward in a width direction (axial direction) is resolved by receiving the guide member 101 provided on the back of the belt end at the end of the extending roller, thereby preventing a shift.
In FIG. 4, when the belt is shifted inward in the width direction (leftward), the guide member 101 and the roller end 101a make contact with each other at an A portion. With a shifting force, the guide member 101 receives a force of mounting on a circumferential surface of the extending roller 110. Therefore, the force concentrates on the A portion of the belt. The guide member 101 receives this force while being wound around the extending roller 110, and is released from this force when being separated. However, a load occurring due to this repeated force on the A portion is so significant that a crack tends to occur on the intermediate transfer belt 100 from the A portion as a starting point. That is, the intermediate transfer belt 100 receives the force mainly at the guide member end A and, when the belt is in a state of slightly mounting on the circumferential surface of the roller or a state of being separated from the roller, the shifting force is lost. With the repetition of load and separation of the shifting force, a crack tends to occur on the belt.
As depicted in FIG. 5, which is a drawing of the structure of another conventional example, to prevent a crack of the belt occurring by the force on the guide member 101, a rear-side reinforcing member (reinforcing tape) 105 is provided on the back of the edge in the width direction of the intermediate transfer belt 100, and the guide member 101 is disposed on the surface of the rear-side reinforcing member 105. However, with the provision of the rear-side reinforcing member 105 on the back of the belt end, the reinforcing tape portion when being wound around the roller will bring an effect similar to that in which the circumferential length of the belt is shortened by the thickness. Therefore, since a belt conveying speed is partially different, the intermediate transfer belt 100 may be deformed. To get around this deformation problem, as depicted in the drawing, a roller step portion 111 is provide to decrease an end diameter of the extending roller 110 according to the thickness of the rear-side reinforcing member 105, thereby solving an influence due to the thickness of the reinforcing member. However, with this configuration, a corner portion B of the roller step portion 111 and the intermediate transfer belt 100 rub in a sliding manner, thereby wearing the belt and causing a crack to tend to occur. Moreover, such a belt crack tends to occur from the end in a width direction. Once a small crack occurs at the end of the belt, the crack is spread. Once a crack occurs at the belt end, the rear-side reinforcing member 105 almost cannot prevent the spread of the crack.
To get around this, as depicted in the drawing, a front-side reinforcing member (reinforcing tape) 106 is provided on the front of the belt end. A crack prevention effect of the front-side reinforcing member is stronger than that of the rear-side reinforcing member but, if the width of the front-side reinforcing member is narrower than the width of the rear-side reinforcing member, a crack may occur outside of the reinforcing members, thereby posing a problem in durability.
In a further explanation, different schemes of attaching a reinforcing member to prevent a crack on a belt are used according to the belt characteristics and system. There are a pattern in which a reinforcing member is provided only on the back of the belt, a pattern in which a reinforcing member is provided only on the front of the belt, and a pattern in which reinforcing members are provided on both of the front and back of the belt. Furthermore, there is a pattern in which a belt shift is controlled without providing a reinforcing member.
Effects of preventing the occurrence of a crack of various reinforcing members included in the intermediate transfer belt 100 with its back provided with the guide member 101 are considered to be increased in the order of (1) a rear-side reinforcing member, (2) a front-side reinforcing member, and then (3) reinforcing members provided on both of the front and back surfaces. That is, the effect of preventing the occurrence of a crack is weak with a single rear-side reinforcing member, whilst this effect is maximized with reinforcing members provided on both of the front and back surfaces.
In the case of (1) the rear-side reinforcing member 105, when a crack occurs on the belt end, the crack is spread at the time of extending the belt by the extending roller. Thus, once a crack occurs, an effect of preventing a spread of the crack is weak. Also, with the provision of the rear-side reinforcing member, an area where the reinforcing member is provided brings an effect similar to that in which the circumferential length of the belt is shortened by the tape thickness. A portion away from a portion in close contact with the extending roller 110 occurs, and therefore the intermediate transfer belt 100 tends to be wavy. When the intermediate transfer belt 100 becomes wavy, a detection failure may occur when a toner pattern is formed on the belt and this toner pattern is detected by a sensor. Furthermore, a difference in circumferential length between portions in a belt width direction will bring a difference in belt running speed. That is, the belt speed of the portion where the rear-side reinforcing member 105 is provided is higher than the belt speed of a portion where no reinforcing member is provided. As a result, the belt tends to be shifted inward in a width direction, thereby causing a crease in the belt.
In the case of (2) the front-side reinforcing member 106, the belt end is sandwiched with the guide member 101. Therefore, even if a crack occurs at the belt end, the crack is prevented from spreading. However, since there is no member for reinforcing a joint between the ends in a circumferential direction of the guide member 101, a crack tends to occur at a portion of the belt corresponding to the joint.
In the case of (3) the reinforcing members on both of the front and back surfaces, the both of the front and back surfaces of the intermediate transfer belt 100 are sandwiched by the reinforcing members 105 and 106. Therefore, an effect of reinforcement against a crack from the belt end is strong. However, it is often the case that the width of the front-side reinforcing member 106 and the width of the rear-side reinforcing member 105 are equal to each other or the front-side reinforcing member is shorter than the rear-side reinforcing member, and a step portion formed between an inner edge of the front-side reinforcing member 106 and a belt surface are not covered. Therefore, when reinforcing members are provided on both of the front and back surfaces of the belt, although the crack prevention effect against a crack from the belt end surface is strong, an effect of preventing a crack occurring from a step portion inside of the reinforcing member is weak.
Japanese Patent No. 3210725 discloses a technology in which, in an image forming apparatus that cleans the belt surface with a blade, a rear-side reinforcing tape is provided along the belt edge, and a step is provided at the roller end facing to this reinforcing tape. This conventional technology, however, is to ensure flatness of the belt by providing a step at the roller end for blade cleaning, and no mention is found about the occurrence of a crack due to the step and its prevention or about a front-side reinforcing tape.
Japanese Patent No. 3079764 discloses a structure in which a reinforcing tape is provided on each of the both of the front and back surfaces of a belt. However, the width of the front reinforcing tape is equal to the width of the back reinforcing tape, and a step corresponding to the reinforcing tape width is not provided on a roller side. Therefore, an effect of preventing a crack occurring from a step between an inner edge of the reinforcing tape and a belt surface cannot be expected.
Japanese Patent Application Laid-Open No. 11-219046 relates to a front-side reinforcing tape. However, only a guide member is provided on the back of a belt, and no rear-side reinforcing tape is provided. Therefore, no step portion is provided on the roller. Therefore, an effect of preventing a crack occurring from a step portion between an inner edge of the front-side reinforcing tape and a belt surface cannot be expected.
Furthermore, no explanation about a crack occurrence portion and a reinforcing tape requiring crack prevention is present in any of the conventional technologies mentioned above.